Time Machine Drive Unmountable After Big Sur → Tahoe Backup (APFS Container Corruption)

[mtthwjrd]

I’m dealing with a Time Machine drive that became unreadable after backing up across two macOS generations and I’m hoping to restore the APFS container or at least extract data with directory structure intact.

Drive: 4TB WD Easystore (External USB)
Original Machine: MacBook Pro (2014) - macOS Big Sur
New Machine: MacBook Air (M1) - macOS Tahoe
Current Machine: MacBook Pro (M5) - macOS Tahoe

This drive was used for years as a Time Machine disk on the 2014 MBP (Big Sur). The backup history was long and fully intact.

I then connected this same drive to my M1 MacBook Air running macOS Tahoe, which uses the new APFS-based Time Machine format, and allowed one backup to run. It appeared to finish successfully. After upgrading to a new MacBook Pro (M5, also on Tahoe), the drive is now no longer mountable on either the new machine or the original 2014 MacBook Pro that had always read it.

Disk Utility Behavior:

• Physical disk shows normally.
• APFS container appears but is listed as Filesystem: Unknown.
• APFS volume cannot be mounted.
• First Aid reports:
  • spaceman cib out of order
  • Space manager is invalid
  • container superblock could not be verified

Recovery Attempts So Far:

• Disk Drill can scan the physical disk and APFS physical store and recover raw photos/videos/documents via file signature carving.
• However, no Time Machine directory structure (Backups.backupdb) or user home folder hierarchy is detected.
• Scans of the logical APFS volume return 0 files, which suggests APFS metadata is damaged, but underlying data blocks are intact.

Goal is to recover: ~/Pictures and ~/Music, ideally with filenames and folder structure preserved.

Raw file recovery is acceptable if we have no other option but I want to exhaust APFS container/snapshot reconstruction options first.

Has anyone had success rebuilding or extracting APFS Time Machine snapshot metadata after a mixed Big Sur → Tahoe Time Machine write?

Specifically:

• Is it possible to repair or reconstruct the APFS container superblock/checkpoint sufficiently to mount read-only?
• Is Snapshot metadata (ds_snapshot / role=Tm) realistically recoverable in this scenario?
• Or is this effectively a straight file-carving recovery case?

No drive hardware errors / no SMART failures. This appears to be purely logical APFS container corruption caused by the cross-version Time Machine format change.

Thanks in advance; this is my first MacRumors post.